This invention pertains to a process for the cyclodimerization of 1,3-butadiene or substituted 1,3-butadienes to 4-vinylcyclohexene or a substituted derivative thereof.
4-Vinylcyclohexene (hereinafter referred to as vinylcyclohexene) and substituted vinylcyclohexenes are useful starting materials for the synthesis of styrene and substituted styrenes. Styrene is a well-known monomer for polystyrene plastics and composites.
Vinylcyclohexene can be prepared by an uncatalyzed thermal dimerization of butadiene. Disadvantageously, this process yields a mixture of products including vinylcyclohexene, 1,5-cyclooctadiene and polymeric butadienes, which must be separated to obtain the desired vinylcyclohexene product.
Catalyzed processes are also known for the dimerization of butadiene. For example, British Patent 1,554,942 and U.S. Pat. No. 4,125,483 disclose a process for the catalytic dimerization of butadiene to vinylcyclohexene in the presence of a cation-exchangeable aluminosilicate into which copper(I) ions and ions of an alkali metal having an atomic number of at least 19, preferably, cesium, have been introduced. The aluminosilicate includes natural and synthetic zeolites, such as faujasite, as well as clay minerals, such as montmorillonite, and other synthetic silica aluminas. It is taught that copper is introduced into the aluminosilicate via ion-exchange with a copper(I) or copper(II) salt.
U.S. Pat. No. 3,444,253 also discloses the dimerization of butadiene to vinylcyclohexene in the presence of copper(I) zeolites X or Y. The catalyst is taught to be prepared by ion-exchange of sodium zeolite X or Y with cuprous iodide in liquid ammonia or by the reduction of copper(II) zeolite X or Y with carbon monoxide, ammonia, acetylinic hydrocarbon or an olefinic hydrocarbon.
U.S. Pat. No. 4,665,247 relates to a process for the cyclodimerization of butadiene to vinylcyclohexene under Diels-Alder conditions in the presence of a copper-containing ZSM-12 zeolite catalyst. It is taught that the ZSM-12 zeolite is ion-exchanged or impregnated with copper(II) cation.
P. Renger F. Janowski, F. Wolf and E. Jahn report in Z. Chem, 19, (1979), 194-195, that butadiene is cyclodimerized to vinylcyclohexene in the presence of silica gel impregnated with copper(II) ions.
All of these processes suffer from the same manifold disadvantages. First, and most importantly, the lifetime of these catalysts is short, and the catalyst easily deactivates from coking and fouling. second, the preparations of the catalysts are difficult and expensive. For example, the catalysts prepared by ion-exchange with copper(II) salts must be reduced to the copper(I) oxidation state, which is the active form of the catalyst. Disadvantageously, the reduction process is inefficient. Alternatively, the catalysts may be prepared without reductants by ion-exchange with copper(I) salts; however, this route is disadvantageous because copper(I) salts oxidize easily and are not readily solubilized without expensive solubilizing ligands. Third, the regeneration of these catalysts typically requires burning off the coked material at high temperatures, typically at least about 400.degree. C.; however, such a procedure oxidizes copper(I) to copper(II), and therefore a reduction procedure is again necessitated to bring the catalyst back into the active cuprous form. Finally, in certain instances the catalysts may possess low activity and even low selectivity.
It would be desirable to have a highly active catalyst for the dimerization of butadiene to vinylcyclohexene. It would be advantageous if such a catalyst could be easily prepared without expensive solubilizing ligands and without an additional and inefficient reduction step. It would be even more advantageous if such a catalyst did not rapidly coke and deactivate, but rather possessed a long lifetime. Finally, it would be most advantageous if such a catalyst could be regenerated easily and without a separate reduction procedure.